Note: Descriptions are shown in the official language in which they were submitted.
~- ` PlrN ~5~l~
~ Zo BKS/YMB/~OPP
"Magneto-resistive reading head".
The inventlon relat~s to a magnetic reading head
for detecting information-representing magnetic fields on a
magnetic recording medium, comprising a magneto-resisti~e
element of metallic ferromagnetic material which is supported
on a substrate~ which has an easy axis of magnetization in
the plane of the element and which is provided with electrical
contacts for enabling it to be connected to a source of Measur-
ing current, means which force the measuring current to flow
through the element at an angle of at le~t 35 and at most
55 with the easy axis of magnetization, and a magnetic
device for subjecting the element to a magnetic auxiliary
field which is parallel to the easy axis of magnetization.
A head of the above-men-tioned kind is described
in "The Barberpole~ a linear magneto-resistive head" in
IEEE Transactions on Magnetics, September 1975~ Vol. MAG 11
No. 5, pp 1215-1217.
This paper describes a head using a strip-shaped
element of a ferro-mag~etic metallic material having low
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anisotropy, for example Ni-~e, which is held with one of its
edges in the immediate proximity of, or in contact with, a
magnetic recording medium. The fields on the recording medium
produce variations in the magnetization of the element when the
medium is moved across the head and modulate the resistance
thereof ~ia the magneto-resistance effect. Thus, ~len -the
recording medium passes the head, the information~representing
magnetic fields present on the medium rotate the spin system
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PIIN S5~4
7-~) 197~
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of the magneto-resistive element to cause a variation in the
electrical resistance pf the element. When a detection circuit
is conn.ected to the element then an output signal from this
circuit can be a function of the~lnformation stored in the
recording medium.
Since the variati.on in the resistance Or a magneto-
resist-ive element under the influence of an external rnagnetlc
field is quadratic, it is usual in reproducing analogically
recorded signals to optimize the operation of the head by
linearizat-ion of the resistance-magnetic field characteristic.
For this purpose, -the magnetization direction of the element
in the presence of a signal field of zero s-trength should be
.caused to make an angle of approximately 45 with the direction
of current flow through the element~
. With the magneto-resistive reading head described
. in the paper this is achieved by melting the easy axis of
magnetization l.ie parallel to the largest dimension o~ the
element and providing bias means which force the current to
flow through the element at an angle of approximately 45
with th.e longitudinal direction.
The head of the paper furtherrnore comprises a
magnetic device for producing a magnetic auxiliary field in a
. direction parallel to the easy axis of magnetization of the
element. The use of the auxiliary field ensures that one of
the two opposite directions along which the magnetization
vector may orient itself is favoured more than the other
direction so that reversal froln one direction to the other9
which woùld result in a vari.ation of the resistance of the
element~ i5 prevented.
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PlIN ~55l(
, 7-~J-197'7
2~3
In magneto-resistive heads of the above-described
kind~ the output signal V may be written as V = f (Cy , h, Ho),
wherein
~ is the angle between the magnetization and the current
direction with a signal field of zero strength, h is the s:ignal
field strength of the recording medium, and Ho is a character~
istic quantity for a given magneto-resistive element which
may be writtén as H = (d x 4 ~rM ) + Hk, wherein d is the
thickness of the magneto-resistive element, w is the height,
Ms is the saturation magnetization, and ~ is the anisotropy
field.
With a given average value of h it is usual
to adjust the angle ~ to provide a minimum distorsion of
the output signal. (As already noted, ~ usually has-a value
in the proximity of 4~ ). Once ~ has been adjusted, for
example, by arranging a pa-ttern of equipotential strips or of
slots which force the current to flow at a given ang~e
any chan~e of the angle is no longer possible. The dynamic
range of h, that is the maximum amplitude of h for a per-
missible degree of distorsion, then follows from the character-
` istic quantity Eo which is determined by the geometry and the
physical properties of the selected element. A disadvantage
of this arrangement is that the dynamic range is fixed in
construction of the head and it cannot be changed subsequently
for example~ to adapt the head to the use of di~ferent recording
materials or according to the magnitude of the a~erage
signal which is required to be read out.
It is the object of the invention to provlde a
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PHN 85~4
7-6-1977
11~311ZO
magnetic reading head of the kind mentioned in the preamble
in which the dyna~c range can be adJusted according to the
signal required to be read out.
For tha-t purpose, the ~eading head according to
the invention is characterized in that, for adjusting the
dynami.c range of the element, rneans are present to vary the
strength of the auxiliary f:ield between 0.1 Ho and 5 Ho
wherein Ho = (d x 4 Ms) ~ ~ , wherein d is the thickness
of themagnetoresistive eleme~, w is the height, Ms is the
sat~ration magnetization, and ~ is the anisotropy field.
~he invention is based on the fact that by varying
the strength of the auxiliary field theslope of the
resistance-signal field characteristic of linearized magneto-
, . .
resistive elements of the present kind, that is to say
15 . linearized by non-magnetic means, varies with the operating
y
point remaining the same. This in oontrast with the
behaviour of magneto-resistive elements the characteristic.
of which is linearized by applying a magnetic field at right
angles to the easy axis of magnetization. In magneto-
resistive elements of such a magnetically biased type, the
operating point shifts when the strength of the bias field
is varied, so distorsion occurs.
'' ~f'~ ' ' Although it is possible in non-magnetically biased
;~ . magneto-resistive heads of the k.ind mentioned in ~e preamble
to enforce the d~sired current direction in various manners~
for example, by providing slots in the element which extend
at the deslred angle with respect to the longitudinal direction
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~ 20 7-6-~977
and alternately open into the upper and lower side, a dif~erent
construction is used -n a preferred embodiment o~ the inven-tion.
According to a di~ferent aspect of the invention there is
provided a head in which the easy a~is of magnetization is
parallel to the ~argest dimension of the element and in that
a number of parallel equipotential strips of electrically
conductive material are provided on a major surface of the
magneto-resistive element at an angle of at least 35 and
at most 55 with the largest dimension between the contacts.
An advantage of this construction is that in applications in
which the magneto-resisti~Te element -ls in contact with the
recording medium, the improvod operation need not be lost in
tlie event of some surface wear of the element which would be
likely in a construction having slots. In addition, the
magnetic continuity of a magneto-resistive element is not
interrupted when equipotential strips are provided on it. In
a construction having slots on the contrary the magnetic
continuity may possibly become damaged when surface wear
occurs.
The invention will now be described in greater
detail, by way of examplej with re~erence to the accompanying
drawings, but is not restricted to this embodiment.
In the drawings:
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Fig. 1 is a simplified perspective view of a
magnetic reading head according to the invelltion with the
associated circuits in block form.
Fig. 2 shows the individual components of Fig.
1 on an enlarged sca]e.
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3 ~I-IN 855/1
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1~ 0
I~`ig. 3 is a ~raph showin~ the resistance variation
~ R/ ~ RM~X of the magneto-resistive element 3 of the head
shown in ~ig. 1 as a function of a transversal external
field h~ at various values of a longitudinal field hy.
F:ig, 1 shows a magneto-resistive head 1 which is
depicted in use for reading the information content of a
magnetic recording medium 2. The head 1 comprises a magneto-
resistive element 3 which is connected to an external reading
circuit 6 via electrically conductive contacts 4 and 5. The
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element 37 and the contacts 4 and 5 are formed on a substrate
7, which may be glass, by means of thin-Pilm techniques. For
; supplying a measuring current, a current source 8 is connected
to the ContaGt 4 and 5. Fig. 2, in which the same re-Perence
numerals as those in Fig. 1 are used for the same components,
shows the individual components of the head on a larger scale.
In this embodiment to the element 3 was a thin layer of an
Ni-Fe alloy having a thickness d of approximately 0.1 micron,
a le3~gth 1 of 100 microns and a height w o-P 10 microns. The
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contacts 4 and 5 are formed by vapour-deposited gold strips.
A number of thin gold strips 9, thickness 1 micron, width
2 microns, are provided on the element 3 at a mutual distance
of 2.5 microns and at an angle olP 45 The gold has a 5 times
lower resis*ivity than the Ni-Fe alloy used~ and the~-thickness
of the gold strips is approximately 10 times as large as the
thickness d Or the magneto-resistive elernent material. Therefore
the gold strips c03lduct 50 times better and serve as
"equipotential strips" which force the current in the Ni-Fe
path between them to flow at an angle of approximate]v 45
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PHN ~r~5ll
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with the longitudirlal d-irection. When t;he head is brought in
flux coupling with an information-containillg magnetic field,
the resistance of eacll of the ~i-Fe paths situated between
the equipotential strips 9 will decrease or increase in
accordance with the fact whether the direction of magncti~ation
under the influence of the field coincides rnore or less with
the direction of current. In this manner, magnetic recordings
can be reproduced with 10w distortion by means of a non-
magnetically biased magneto~resistive head.
A longitudinal auxiliary field 3-I~ is generated
by means of a magnet core 10. The strength of aid auxiliary
field can be varied between 0.1 Ho and 5 Ho by varying a
current through a coil 11 which is connec-ted to a variable
current source 12, IIo being a characteristic quantity, as
; 15 defined above, for the dimensions and material of the element
in question. Said field strength may be such that it is
unlittely to damage the stored information on the medium 2.
It is alternatively possible to use a permanent magnet which
is magnetized parallel to the longitudinal direction of the
magneto~resistive element and which is supported at a
variable distance from the element, The use of a permanent
magnet i~ illustrated diagrammatically in Fig. 3 wllichs~ows
a substrate 13 on which a non-magnetically biased magneto-
resistive element 14 is pro~7ided and a magnet 16 which can be
-~ 2~ moved by means of a screw 15. In or~er to illustrate the
effects of the variation of the au~iliary field, Fig. 4
j shows the variation ~ R/ ~ Rma~ of the resistance of the
elemellt 3 under the iDfluence of` a sienal field Hz
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~ ~'MN 8554
~ ZO 7-~-1977
with various strengths of an au~iliary ~ield h~, in the +y
direction. It is to be noted that both the signal field h
and the auxiliary field hy are standardized~ that is
hz = ~z/Ho and hy = Hy/H , where Mz and My are the actual
signal -~leld stren~th and the actual longitudinal field
strength, respectively. The characteristic quantity Ho
which depends inter alia on the ratio height/thickness w/d
in practive proves to be usually between ten and a hundred
Oersteds.
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